Arched windows and doorways add a touch of elegance to any home, but they present a unique challenge for finish carpenters: how do you create curved casing that matches the straight casing already installed throughout the house? One highly effective solution is strip lamination, a technique that involves ripping molding into thin strips and bending them around a curved form. This method allows builders to produce custom curved casings on site that perfectly match existing profiles. Understanding why miter joints sometimes open on window and door casings helps appreciate the importance of proper joinery in curved applications, where movement and seasonal expansion are amplified.
Understanding Strip Lamination for Curved Casing
Strip lamination is a woodworking technique that bends thin layers of wood around a form to create a curved shape. Unlike steam bending or kerf cutting, which weaken the wood, strip lamination preserves strength by distributing stress across multiple layers. Each thin strip flexes easily without breaking, and when glued together, the laminated assembly holds its curved shape permanently.
How Strip Laming Works
The principle is simple. A straight piece of casing molding is ripped lengthwise into several thin strips, each about 1/8 to 3/16 inch thick. These strips are then coated with glue, stacked together, and clamped around a curved bending form. Once the glue cures, the strips become a single solid piece of curved casing that retains the original profile.
Advantages Over Other Methods
- Strength: Laminated curves are stronger than solid wood cut to shape because the grain follows the curve.
- Consistency: The casing profile is reproduced exactly from the original straight molding.
- On-site fabrication: No need for specialized equipment or off-site milling.
- Matching profiles: Alternating strips from two identical pieces of straight casing recreates the original profile seamlessly.
- No waste: Unlike cutting a curve from a wide board, strip lamination uses every part of the casing material.
Step 1: Making Patterns for Arched Openings
Before any cutting or laminating begins, accurate patterns of each arched opening must be created. The approach varies depending on the complexity of the arch and the tools available.
Simple Tracing Method
For standard arched windows, kraft paper can be pressed directly against the frame and the arch traced by hand. Kraft paper comes in long rolls and is ideal for this application. The traced line is then cut out and checked against the opening.
Flexible Strip Method
For more complex arches, a thin, flexible strip of even-grained wood is clamped against the inside of the frame to follow the arch. Crosspieces are tacked to the bowed strip to maintain the shape after the clamps are removed. The more crosspieces used, the more accurate the shape retention. This captured shape is then transferred to kraft paper for the final pattern.
Mathematical Computation Method
Some arches require mathematical computation to determine radius points, especially for multiple-radius or elliptical arches. A plywood panel is tacked against the window frame, and a beam compass is used to find the radius points by trial and error. This method is more time-consuming but essential for non-standard arch shapes.
Best Practice: Pre-Installation Tracings
The ideal time to create patterns is before the window frames are installed. Once the frames are in place and drywall is finished, access becomes more restricted. Making tracings during the rough-in stage saves significant time and effort later.
Step 2: Building Bending Forms and Preparing Strips
With patterns ready, the next phase involves constructing bending forms and preparing the casing strips.
Constructing the Bending Forms
Bending forms are made from 2x lumber cut into arcs using a bandsaw. For roundtop casings, a circle-cutting jig fixed to the bandsaw table works well. The table is extended with a piece of 1/4-inch plywood, and a screw serves as a pivot point. The 2x stock rides on a plywood carriage and pivots around the screw to cut a precise arc.
For larger windows with gradual arches, the pivot point may need to be extended across the shop using a long 1×3 board as a guide. The finished 2x arcs are screwed to a plywood base or directly to the subfloor, depending on size.
Preparing the Casing Strips
Two identical pieces of straight casing are selected, ensuring they have even grain and are free of defects. Each piece is ripped lengthwise into thin strips on a table saw. Using a high-quality blade and proper technique is critical at this stage. Following table saw safety regulations and best practices for woodworking ensures that this step is completed without injury and with clean, accurate cuts.
The strips from the two casing pieces are then alternated when stacking, which helps the glue bond more evenly and reproduces the original casing profile accurately. Each strip is numbered or labeled to maintain the correct sequence during assembly.
Recommended Strip Dimensions
| Application | Strip Thickness | Number of Strips | Glue Type |
|---|---|---|---|
| Small roundtop windows | 1/8 inch | 5 to 7 | Yellow wood glue |
| Standard door arches | 3/16 inch | 3 to 5 | Waterproof PVA |
| Large arched windows | 1/8 inch | 7 to 10 | Epoxy or polyurethane |
| Elliptical or compound curves | 1/16 to 1/8 inch | 10 to 15 | Epoxy |
Step 3: Laminating and Installing Curved Casing
The lamination process requires careful preparation and patience, but the results are well worth the effort.
Applying Glue and Clamping
Each strip is coated evenly with glue on both sides. The strips are stacked in the predetermined sequence on the bending form. Clamps are applied starting from the center of the arch and working outward to prevent glue squeeze-out from trapping air pockets. Cauls (thick wooden blocks) placed between the clamps and the strips help distribute clamping pressure evenly across the width of the casing.
Curing and Finishing
The assembly should remain clamped for at least 24 hours at room temperature. After the glue cures, the laminated casing is removed from the form and any excess glue is scraped away. The casing is then sanded smooth and finished to match the rest of the trim work. At this stage, the casing can also be cut to final length with mitered ends for joining at the spring points of the arch.
Installation Considerations
Installing curved casing requires careful attention to alignment. The casing should be checked against the opening before any nails or adhesive are applied. Shims may be needed where the wall surface is not perfectly flat. For wide casings, pilot holes should be predrilled to prevent splitting. Understanding how architrave profiles are designed and installed provides useful context for integrating curved casing with straight trim elements.
Laminate Extension Jambs
The same bending forms used for the curved casing can also be used to laminate curved extension jambs. Extension jambs fill the gap between the window frame and the finished wall surface. By laminating them at the same time as the casing, perfect alignment between the jamb and casing is ensured. The same strip lamination technique applies: thin strips of jamb stock are glued and clamped around the form.
Common Challenges and Solutions
Even experienced finish carpenters encounter difficulties when working with curved casing. Knowing what to expect helps avoid costly mistakes.
Spring-Back After Unclamping
If the laminated casing springs back slightly after removal from the form, adding one or two extra strips or using thicker strips may help. Allowing the assembly to cure longer (48 hours instead of 24) also reduces spring-back.
Glue Starvation in Tight Curves
Tight-radius curves can squeeze out too much glue during clamping, leaving dry spots. Applying glue generously and using epoxy or polyurethane glue (which fills gaps better than standard wood glue) solves this issue.
Profile Mismatch
If the laminated casing profile does not match the straight casing, the issue is usually in the strip sequence or the alignment of strips during clamping. Carefully numbering strips and using alignment marks on the form prevents this problem.
Material Selection
Not all wood species bend equally well. Straight-grained woods such as oak, poplar, and mahogany perform best for strip lamination. Woods with interlocked or spiral grain tend to break during bending. When working with difficult species, composite trim for decks and porches offers an alternative material option that can be milled to similar profiles with greater dimensional stability.
Tools and Materials Checklist
- Straight casing molding (two identical pieces per arch)
- Table saw with fine-tooth blade for ripping strips
- Bandsaw for cutting bending form arcs
- 2x lumber for bending forms
- Plywood or subfloor for form base
- Kraft paper for patterns
- Beam compass for radius computation
- Wood glue, epoxy, or polyurethane adhesive
- Bar clamps or strap clamps (at least 6 per arch)
- Cauls for even pressure distribution
- Sanding equipment and finishing supplies
- Shims and pilot drill bits for installation
Strip lamination is a time-tested technique that enables finish carpenters to produce elegant, durable curved casing for arched openings. With careful pattern-making, precise form construction, and attention to gluing and clamping procedures, any builder can achieve professional results. For those tackling window-related trim work, understanding proper window replacement design principles for homeowners and builders helps ensure that the entire assembly functions correctly and looks cohesive.